A turbomachine comprises several turbine stages each comprising an upstream guide vane element formed of an annular array of fixed stator blades and an impeller mounted rotatably downstream of the upstream guide vane element in a cylindrical or frustoconical shroud formed by ring sectors placed circumferentially end-to-end. The first of these stages is a high-pressure stage and the other stages situated downstream are low-pressure stages.
It is important that the radial clearances between the impellers and the corresponding ring sectors be optimized in order to enhance the performance of the turbomachine and to prevent any friction of the blade ends on the ring sectors, which would result in these ends wearing and in the performance of the turbomachine deteriorating at all operating speeds.
The ring sectors that surround the impeller of the high-pressure stage comprise, at their upstream and downstream ends, coupling means interacting with corresponding means provided on an annular support placed between the ring sectors and the turbine casing.
In operation, the hot gases leaving the combustion chamber of the turbomachine flow through the upstream guide vane element of the high-pressure stage and exert thereon an axial pressure in the downstream direction. This upstream guide vane element tends to move in the downstream direction and to press via its outer periphery on the annular support of the ring sectors and to push it in the downstream direction, which causes variations of the radial clearances between the movable blades of the impeller and the ring sectors.